The invention is in the field of imaging using penetrating radiation and pertains in particular to obtaining and processing penetrating radiation measurements and especially to morphometric x-ray absorptiometry referred to by the acronym MXA.
In fields such as the diagnosis of osteoporosis, it can be desirable to confirm a fracture associated with low bone material density, such as a hip, wrist or vertebral fracture [1]. (Numbers in square brackets refer to items which are listed in a bibliography at the end of the specification and are hereby incorporated by reference). Lateral thoracic and lumbar spine films have been utilized for the diagnosis of vertebral fractures in order to confirm crush and wedge deformities of vertebral bodies in the range encompassing T4 and L4 vertebrae. A number of studies have evaluated methods for the identification of vertebral fractures by vertebral morphometry and the correlation thereof with readings of radiologists [2, 3, 4, 5, 6, 7, 8, 9]. Some vertebral morphometry techniques involve digitizing conventional radiograms (x-ray films) and obtaining anterior, posterior and mid-vertebral heights. However, there can be disadvantages in this approach such as operator imprecision in placing the points for digitization on the radiograms, the use of multiple exposures to image both thoracic and lumbar regions of the spine due to the relatively large attenuation difference between the thoracic and lumbar areas, the possible need for retakes, and the radiation dose that can be associated with this procedure (such as 900 mRem without repeat exposures). In addition, geometric distortion can be a factor in using such digitized conventional x-ray films because they typically are obtained using cone beam geometry. As a result of such geometric distortion, different points in the radiogram are magnified and distorted in relative position in different ways. For example, areas closer to the edge of the film image are magnified more and are viewed at a somewhat oblique angle, whereas areas close to the center are magnified less and are viewed at an angle closer to perpendicular. Still in addition, the identification of vertebral levels can be difficult and film handling and archiving can involve considerable overhead. Rectilinear scanning, using a bone densitometer with a thin pencil beam of x-rays can counter the geometric distortion problem but can introduce the disadvantage of a much longer scanning time to acquire the necessary x-ray data. The use of a fan beam CT scanner in a scout view mode can decrease the scanning time as compared with rectilinear scanning. See W. A. Kalender, et al., Determination of Geometric Parameters and Osteoporosis Indices for Lumbar Vertebrae from Lateral QCT Localizer Radiographs, 8th International Workshop on Bone Densitometry, Bad Reichenhall, Germany, Apr. 28-May 2, 1991. However, it is believed that the proposed CT images were not dual energy images and that the proposal may not completely address the issues of geometric distortions and/or vertebral magnification factor differences as between the AP and lateral images. Moreover, it is believed that QCT (quantitative computerized tomography) so used in morphometry typically images a relatively limited region of the spine such as the T12 through L4 vertebrae.
When bone densitometry equipment is used to obtain penetrating radiation images useful in morphometry, typically a patient is placed on a table and remains stationary while a radiation source moves relative to the patient position. A radiation detector is positioned on the opposite side of the table from the source to detect radiation transmitted through the patient. The radiation source and detector are usually mechanically linked by a structure such as a C-arm to ensure alignment between them. Both x-ray tubes and isotopes have been used as a source of the radiation. In each case, the radiation from the source is collimated to a specific beam shape prior to reaching the patient to thereby restrict the radiation field to the predetermined region of the patient opposite which are located the detectors. In the case of using x-rays, various beam shapes have been used in practice or proposed, including fan beam, pencil beam and cone or pyramid beam shapes.
Bone densitometry systems are manufactured by the assignee hereof under tradenames including QDR 2000plus, QDR-2000, QDR-1500, QDR-1000plus, QDR-1000W and QDR-1000. Certain information respecting such equipment can be found in brochures originating with the assignee hereof and identified by the designators B-108 (9/93) USA, B-109 (9/93) USA, S-117 (9/93) USA and S-118 (10/93) USA. Commonly owned U.S. patents pertaining to such systems include U.S. Pat. Nos. 4,811,373, 4,947,414, 4,953,189, 5,040,199, 5,044,002; 5,054,048, 5,067,144, 5,070,519, 5,132,995 and 5,148,455 as well as 4,986,273 and 5,165,410 (assigned on its face to Medical & Scientific Enterprises, Inc. but now commonly owned). Commonly owned U.S. patent application Ser. No. 08/156,287 filed on Nov. 22, 1993 also pertains to a bone densitometer. Said patents and application and said brochures are hereby incorporated by reference herein. Other bone densitometry systems are believed to be offered by other companies, such as the Lunar Corporation of Madison, Wis. See, e.g., J. Hanson, et al., New Imaging Bone Densitometer, Presented at: The American Society for Bone and Mineral Research 15th Annual Meeting, 18-22 Sep., 1993, Tampa, Fla., USA, an undated flier entitled Product Information EXPERT, and U.S. Pat. No. 5,228,068, none of which is necessarily admitted to be prior art against the invention claimed in herein. Note the discussion of an approach to morphometry in said U.S. Pat. No. 5,228,068.
For a general background concerning MXA, see Morphometric X-Ray Absorptiometry (MXA), a document prepared by the assignee hereof and identified by the designation W-126 (10/93) USA, which is hereby incorporated by reference.